1. Field of the Invention
The present invention relates generally to a television signal receiving system. In particular, the present invention relates to an automatic frequency tracking apparatus and a method for a television signal receiving system which can automatically track the frequency offset of a received digital television signal even in a case where the frequency offset deviates from a determined range in which carrier restoration is possible.
2. Description of the Related Art
In receiving a digital television signal, there usually exists a frequency offset between various frequency oscillators used in transmitting a television signal and a frequency synthesizer of a receiving tuner. In an ideal case, since an allowable error of the oscillators used in transmitting/receiving the television signal is within the range of several kilohertz (KHz), the frequency offset of the received signal detected in a television receiver exists within a range of several tens of KHz. In the actual television radio wave transmitting/receiving environments, however, the frequency offset of the received signal may be in the range of several hundreds KHz due to deterioration of the appliance or parts thereof. Accordingly, a digital television signal receiving system is required to track and compensate for the frequency offset of the received signal.
Frequency tracking is the technique of tracking and compensating for frequency offset if the received signal deviates from the original received frequency and where the frequency offset is due to various reasons during transmission of television signal. Automatic frequency tracking (AFT) methods, which have been adopted in the television receiver, may be classified according to the television receiving system as follows.
In a general analog television receiver, a voltage signal is produced in proportion to the difference between the frequency value of the visual carrier of the received signal and the frequency value of the frequency synthesizer in the receiving tuner, which are in an intermediate frequency (IF) band., The voltage signal is used to determine the value of the frequency offset of the present received signal. Then, the detected frequency offset value is transferred to a microcomputer in the receiver, so that the microcomputer compensates for the frequency offset by adjusting the frequency of the frequency synthesizer to reduce the frequency offset value. In the general analog television receiver, an allowable range of the frequency offset in which tracking is possible is xe2x88x921.5 MHzxcx9c2.0 MHz.
A vestigial side band (VSB) type television receiver currently proposed in the United States has a frequency offset compensating circuit as shown in FIG. 1, and performs the frequency tracking as mentioned above.
The general frequency offset compensating circuit of FIG. 1 has already been proposed by Zenith, and is disclosed in xe2x80x9cGuide to the Use of the ATSC Television Standard A/54xe2x80x9d, published by United States Advanced Television System Committee in 1995. According to the frequency offset compensating circuit of FIG. 1, the received signal is first demodulated to a baseband signal, and the frequency offset value is computed using an AFC low pass filter (LPF) 36 and an automatic phase control (APC) LPF 42 for filtering the baseband signal. Then, the frequency tracking and carrier restoration are performed completely by varying the frequency of a double-conversion tuner 12 in accordance with the output value of the APC LPF 42.
FIG. 2 is a block diagram illustrating a partial construction of a general digital television receiver which restores the carrier with the digital compensation of the frequency offset. Such a digital television receiver is disclosed in detail in Korean Patent Application No. 95-20772 filed by the applicant of the present invention. According to the digital television receiver of FIG. 2, since the carrier restoration is performed after an analog-to-digital (A/D) conversion of the television signal, the oscillation frequency of a second local oscillator 80 in the double-conversion tuner 12 is fixed, and the frequency offset is compensated for by a numerically controlled oscillator (NCO) 68 provided following an analog-to-digital converter (ADC) 56.
As described above, various kinds of automatic frequency tracking methods are employed in the television receiver. However, all the methods have drawbacks in that their carrier-restoration performance is superior in a case where the frequency offset value is relatively small, but it is degraded considerably in a case where the frequency offset value becomes greater than xc2x1300 KHz. Specifically, if the frequency offset value becomes greater, a surface acoustic wave (SAW) filter in the receiver suffers from the loss of the received signal, and thus the carrier restoration itself cannot be performed.
It is an object of the present invention to solve the problems involved in the related art, and to provide an automatic frequency tracking apparatus and method for a digital television signal receiving system which can automatically track the frequency offset of a received digital television signal even in a case where the frequency offset deviates from a determined range in which the carrier restoration is possible.
It is another object of the present invention to provide an automatic frequency tracking apparatus and method for a digital television signal receiving system which can improve the receiving performance of the system.
In order to achieve the above objects, there is provided an automatic frequency tracking method for a television signal receiving system for restoring a carrier including a pilot signal, comprising:
a window region dividing step of dividing a maximum frequency offset range which can occur in receiving a television signal into a plurality of window regions;
a wide-range tracking adjustment mode performing step of searching whether a lock signal of a frequency phase-locked loop is detected at representative frequencies of the respective divided window regions; and
a fine-tuning adjustment mode performing step of determining the respective representative frequencies existing in the window regions in which the lock signal is detected as tuned frequencies, and if a frequency offset detected from the determined tuned frequencies has a value in a predetermined range, determining the corresponding tuned frequency as an optimum tuned frequency.